1. Field of the Invention
The present invention relates generally to a traction control system for an automotive vehicle, for detecting wheel slippage, such as spinning, skidding and so forth, to adjust driving torque to apply on vehicular wheels. More specifically, the invention relates to an automotive traction control system which can precisely detect vehicle speed. Further particularly, the invention relates to a traction control system which is applicable for controlling traction of four-wheel drive vehicle.
2. Description of the Background Art
In general, wheel slippage is reflected in a difference of an actual vehicle speed and an assumed vehicle speed at the vehicle derived based on a wheel speed. When the actual vehicle speed is lower than the assumed vehicle speed, it means that the driven wheel is spinning. When the actual vehicle speed is higher than the assumed vehicle speed, it means that the vehicle wheel causes skidding. Wheel-spinning is caused by loss of road/tire traction. Therefore, in such case, traction control has to be performed in order to prevent the vehicular wheel from spinning. Wheel-skidding occurs during application of abrupt brake and is caused by locking of the wheel. For example, such traction controls have been disclosed in U.S. Pat. No. 3,893,535, issued on Jul. 8, 1975, to M. H. Burckhardt, et al. and in Japanese Patent First Publication No. (Tokkai Showa) 59-68537, published on Apr. 18, 1984. In both cases, a rotation speed of a driven wheel which is driven by an engine output is compared with a rotation speed of a non-driven wheel which rotates freely. The rotation speed of the non-driven wheel is treated as a parameter reflecting an actual vehicle speed.
The detection of the wheel slippage as disclosed in the aforementioned prior art is not applicable for anti-skid or traction control for a four-wheel drive vehicle.
Furthermore, when traction control is performed in a substantially precise manner upon starting the vehicle running on a slippery muddy road, driving torque tends to be reduced to a at which no wheel-spin occurs for any wheels. This results in a lack of driving torque which makes the vehicle impossible to run. In order to allow the vehicle to start-up on a slippery or muddy road, precision of the traction control has to be lowered. Lowering of precision will allow wheel-spin on the wheels. This degrades drivability when the vehicle is running at relatively high speed.